Circuit for operating a discharge lamp

ABSTRACT

The invention concerns a circuit for operating a discharge lamp with a  habridge generator (T1, T2) wherein a turn-off transistor (T3) can stop the half-bridge generator The energy source for the base current of the turn-off transistor (T3) is formed by starter capacitor (C3) which starts the half-bridge generator (T1, T2), In addition, the collector-emitter section of the turn-off transistor (T3) is directly between the base of the lower transistor (T2) of the half-bridge generator and earth.

BACKGROUND OF THE INVENTION

The invention concerns a circuit arrangement for the operation of adischarge lamp.

In the operation and upon ignition of a discharge lamp, high voltagescan occur in a defective lamp and towards the end of a lamp's servicelife, which can lead to destruction of the corresponding electronicballast if no suitable countermeasures have been provided. Customarily,a cutoff circuit is used to monitor lamp voltage. As soon as lampvoltage has been higher than a particular value for longer than apredetermined time, the cutoff intervenes and the lamp generator, whichis generally a half bridge generator, is stopped.

Electronic ballasts with a cutoff circuit from the OSRAM GmbH company(product designation QT EC) are already known. A circuit arrangementcorresponding to this state of the art is shown in FIG. 1. In thisarrangement, the cutoff device is formed from a bistable flip-flopcircuit consisting of two transistors T1 and T2 (thyristor equivalentcircuit). The lamp voltage U_(L) is fed via a voltage divider R4, R3 anda time-lag device R4, Ct to the base of one of the transistors of thebistable flip-flop circuit, which forms the trigger input of thiscircuit. When the bistable flip-flop circuit trips to its conducting oroperation state, a cutoff transistor T3 stops the lamp generator, whichis a half bridge generator in the example described, by suppressing thebase drive of the lower transistor Tu of the half bridge generator via adiode D1. A starting capacitor Cs, which is located parallel to theswitching path of the lower transistor Tu of the half bridge generatorvia a diode D4, is short-circuited via a diode D2 so that renewedstarting attempts by the inverse rectifier are prevented.

This known circuit has two disadvantages in particular. In order thatthe oscillation of the half bridge generator can be dependablyinterrupted, the base of transistor Tu of the half bridge circuit mustbe connected with low impedance to ground. However, since the basevoltage of transistor Tu of the half bridge generator can also shownegative values, the base of this transistor Tu of the half bridgegenerator is connected via a diode D1 to the collector of the cutofftransistor T3. In this way, however, dependable disconnection is mademore difficult, since the base of the lower transistor Tu is notdirectly connected to ground upon disconnection, but only via the diodepath from D1 via T3.

A further disadvantage of the known circuit arrangement is that anenergy source U_(v) must be made available to provide the base currentfor the cutoff transistor T3. In known circuit arrangements, anelectrolytic capacitor is provided for this purpose, which is loadedeither using the trigger signal or via a pump circuit.

Therefore, the purpose of the present invention is to make a circuitarrangement available for the operation of a discharge lamp, in whichthe aforementioned problems are avoided.

SUMMARY OF THE INVENTION

The invention as shown in FIGS. 2 and 3 concerns a circuit for operatinga discharge lamp with a half-bridge generator. The energy source for thebase current of a turn-off transistor (T3) is formed by startercapacitor (C3) which starts the half-bridge generator. The realizationof the circuit arrangement according to invention according (FIG. 2) hasthe advantage that no additional source of energy is required for thecutoff process, but rather that this energy source is provided by thealready present starting capacitor C3. In the previously mentioned knowncircuit, the starting capacitor is discharged when the half bridgegenerator is in operation, via a diode D4 which is located parallel tothe switching path of transistor T2 of the half bridge generator.

By insertion of a Zener diode D9, the starting capacitor C3 remainsloaded at the value of the Zener voltage of the Zener diode and is usedin this way as an energy source. If the bistable flip-flop circuit T5,T4 becomes conductive, not only is the lamp generator stopped, but arestart by discharge of the starting capacitor is also prevented.

The preferred circuit arrangement has the advantage that dependabledisconnection is guaranteed, since the collector-emitter path of cutofftransistor T3 is located directly between the base of transistor T2 ofthe half bridge generator and ground. By using a diode D12 between thecollector of transistor T5 and the base of transistor T4, which formsthe trigger input of the bistable flip-flop circuit, negative collectorcurrents in the cutoff transistor T3 are prevented when negative basevoltages are present in transistor T2 of the half bridge generator.Furthermore, an additional diode D11 can be provided for voltagematching of the base terminals of transistor T4, which forms the triggerinput of the bistable flip-flop circuit, and the cutoff transistor T3.

A current supply for illuminated displays, which contains asingle-transistor combinational circuit component, is known fromWO-A-94/06262. Circuit arrangements for discharge lamps are described inthe documents U.S. Pat. No. 4,554,487 and DE-A-36 26 209, which eachshow a half bridge generator and a cutoff transistor. Triggering of thecutoff transistor is performed here using a diac.

In addition, a circuit arrangement for the operation of electric lampsis described in EP-A-0 648 068, in which the half bridge generator canbe stopped using a field effect transistor. Drive of the field effecttransistor takes place here using a bistable multivibrator.

Additional advantages, elements, and possible applications of theinvention arise from the following description of a sample execution ofthe invention in connection with the drawing.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawing,

FIG. 1 shows a circuit arrangement according to the state of the art,

FIG. 2 shows an initial sample execution of a circuit arrangementaccording to invention, and

FIG. 3 shows a second sample execution of a circuit arrangementaccording to invention.

BEST MODE FOR CARRYING OUT THE INVENTION

FIG. 2 shows a circuit arrangement for the operation of a discharge lampwith a half bridge generator, which is formed from a lower transistor T2and an upper transistor T1. A load circuit with a discharge lamp E (notshown in FIG. 2) is located parallel to transistor T2. The lamp voltageU2 is fed via a voltage divider R13, R15 and a time-lag device R13, C13to the base of a transistor T4, which forms the trigger input of abistable flip-flop circuit from a first and second transistor T4 and T5,respectively. When the flip-flop circuit is in a conductive state, acutoff transistor T3 stops the half bridge generator by suppressing thebase drive of transistor T2.

The starting capacitor C3 is located parallel to the switching path oftransistor T2 and in series with diode D4 and Zener diode D9. Thestarting capacitor C3 is loaded by this Zener diode D9 to the value ofthe Zener voltage and can thus serve as an energy source for the basecurrent of cutoff transistor T3.

In order to start the half bridge generator T1, T2, the startingcapacitor C3 is loaded via resistor R1 until it reaches the breakdownvoltage of a diac DIAC. A base current of the half bridge transistor T2flows via the diac DIAC. In this way, the transistor T2 is switched intothe conductive state, so that a load current flows into the load circuitC10, C11, R8 in FIG. 3 of the discharge lamp E. A coil (RK1-A in FIG. 3)in the load current is coupled to a coil Rk (RK1-C in FIG. 3), which isinserted between resistor R6 connected in series to the base oftransistor T2 and ground. In this way, a self-maintaining oscillation ofthe half bridge generator is triggered. Following stimulation ofoscillation in the generator, C3 is loaded to the Zener voltage of D9.

In addition, a diode D11 can be provided at the base of transistor T3for voltage matching of the base terminals of transistors T4 and T3, aswell as a diode D12 between the collector of the second transistor T5and the base of the first transistor T4 in a circuit arrangement for theoperation of a discharge lamp.

While a diode D1 is provided for in the circuit according to FIG. 1,which serves to prevent a discharge of Ct via the base-collector path ofT3 upon negative base-emitter voltage of transistor Tu, the circuitaccording to invention according to FIG. 2 provides that this diode isomitted. In the circuit according to invention according to FIG. 2,discharge of C13 is prevented by diode D12. In this way, it becomespossible to connect the collector of cutoff transistor T3 directly tothe base of half bridge transistor T2, so that in case of cutoff, thebase of T2 will be connected to ground at a lower impedance (withoutvoltage drop at diode D1 provided for according to FIG. 1).

D11 serves to ensure that a sufficiently large drive current isavailable for T4, so that the entire collector current of T5 does notdischarge into the base of T3.

In this preferred sample execution, two circuit arrangements arerealized in order to solve the aforementioned problems in a circuit.However, they can also easily be used independently of one another.

The circuit arrangement shown in FIG. 3 is similarly structured to thecircuit arrangement shown in FIG. 2, which shows the inventiveprinciple. The same components are labeled with the same referencesymbols.

The circuit arrangement according to FIG. 3 has a not-shown power supplycomponent at terminals 1 (ground), 2, as well as a lamp stabilizationcircuit with capacitors C10, C11, and a posistor R8. The half bridge hastwo recovery diodes D6 and D7. A resistor R9 serves to draw the centertap of the half bridge at the transistors T1 and T2 to a predeterminedvoltage for stimulation of oscillation in the half bridge. A capacitorC5 (a trapezoidal capacitor or "snubber") serves to reduce switchinglosses of the half bridge transistors.

A capacitor C6 functions as an additional resonance capacitor, whichserves to increase the generator output voltage.

Via a diode D10, the lamp voltage is rectified, in order to detect peakvoltages of the lamp voltage.

A capacitor C14 prevents triggering of the bistable flip-flop circuit byparasitic inductions, which may arise e.g. due to crosstalk betweenstrip conductors of the circuit arrangement.

I claim:
 1. A circuit arrangement for the operation of a discharge lamp,preferably a low-pressure discharge lamp, with a half bridge generatorwith an upper transistor (T1) and a lower transistor (T2), to which avoltage proportional to that of a voltage (U_(L)) across the lamp is fedvia a time-lag device (R13, C13) to a base of a first transistor (T4),which forms a trigger input of a bistable flip-flop circuit consistingof the first and a second transistor (T4, T5), whereby a cutofftransistor (T3) switches the half bridge generator off when theflip-flop circuit (T4, T5) is in a conductive state, in that the cutofftransistor (T3) suppresses base drive of the lower transistor (T2) ofthe half bridge generator, and whereby the half bridge generator can bestarted by a starting capacitor (C3), which is discharged duringoperation of the half bridge generator via a discharge diode (D4), withthe featurethat the cutoff transistor (T3) has a collector-emitter pathlocated directly between a base of the lower transistor (T2) of the halfbridge generator and ground, that a diode (D12) is located between acollector of the second transistor (T5) and a base of the firsttransistor (T4) of the bistable flip-flop circuit, and that a diode(D11) is coupled to a base of the cutoff transistor (T3).
 2. A circuitarrangement for the operation of a discharge lamp, preferably alow-pressure discharge lamp, with a half bridge generator (T1, T2), towhich a voltage proportional to that of a voltage (U_(L)) across thelamp is fed via a time-lag device (R13, C13) to a base of a firsttransistor (T4), which forms a trigger input of a bistable flip-flopcircuit consisting of a first and a second transistor (T4, T5), wherebya cutoff transistor (T3) switches the half bridge generator (T1, T2) offwhen the flip-flop circuit (T4, T5) is in a conductive state, in thatthe cutoff transistor (T3) suppresses base drive of a transistor (T2) ofthe half bridge generator, and whereby the half bridge generator (T1,T2) can be started by a starting capacitor (C3), which is dischargedduring operation of the half bridge generator via a discharge diode(D4), with the feature that the starting capacitor (C3) forms an energysource for base current of the cutoff transistor (T3), the transistor(T2) of the half bridge generator has a collector, and a Zener diode(D9) is inserted between the discharge diode (D4) and the collector ofthe transistor (T2) of the half bridge generator.